During the drilling of a wellbore, various fluids are typically used in the well for a variety of functions. The fluids may be circulated through a drill pipe and drill bit into the wellbore, and then may subsequently flow upward through the wellbore to the surface. During this circulation, the drilling fluid may act to remove drill cuttings from the bottom of the hole to the surface, to suspend cuttings and weighting material when circulation is interrupted, to control subsurface pressures, to maintain the integrity of the wellbore until the well section is cased and cemented, to isolate the fluids from the formation by providing sufficient hydrostatic pressure to prevent the ingress of formation fluids into the wellbore, to cool and lubricate the drill string and bit, and/or to maximize penetration rate.
Fluid compositions used for these various purposes may be water- or oil-based and may comprise weighting agents, surfactants, proppants, and polymers. However, for a wellbore fluid to perform all of its functions and allow wellbore operations to continue, the fluid must stay in the borehole. Frequently, undesirable formation conditions are encountered in which substantial amounts or, in some cases, practically all of the wellbore fluid may be lost to the formation. For example, wellbore fluid can leave the borehole through large or small fissures or fractures in the formation or through a highly porous rock matrix surrounding the borehole.
Lost circulation is a recurring drilling problem, characterized by loss of drilling mud into downhole formations. It can occur naturally in formations that are fractured, highly permeable, porous, cavernous, or vugular. These earth formations can include shale, sands, gravel, shell beds, reef deposits, limestone, dolomite, and chalk, among others. Other problems encountered while drilling and producing oil and gas include stuck pipe, hole collapse, loss of well control, and loss of or decreased production.
Lost circulation may also result from induced pressure during drilling. Specifically, induced mud losses may occur when the mud weight, required for well control and to maintain a stable wellbore, exceeds the fracture resistance of the formations. A particularly challenging situation arises in depleted reservoirs, in which the drop in pore pressure weakens hydrocarbon-bearing rocks, but neighboring or inter-bedded low permeability rocks, such as shales, maintain their pore pressure. This can make the drilling of certain depleted zones impossible because the mud weight required to support the shale exceeds the fracture pressure of the sands and silts.
Lost circulation is frequently controlled by including an additive in fluids injected into wellbores. The most common additive used to control or cease lost circulation is bentonite which will seal small holes or fractures. Bentonite, in higher concentrations, increases viscosity and slows the fluid flow into the surrounding rock. Other solids, such as ground paper, ground corn cobs and sawdust, have also been used to control fluid loss. Polymers are also sometimes used to increase the viscosity of a wellbore fluid and to control fluid loss. Polymer additives, however, are generally more expensive than particulates such as bentonite.
The use of fibers and solids to prevent lost circulation during drilling operations has been widely described. Such fibers include, for example, jute, flax, mohair, lechuguilla fibers, synthetic fibers, cotton, cotton linters, wool, wool shoddy, and sugar cane fibers. One known process for preventing or treating lost circulation involves the addition, at concentrations ranging between about 1.43 and about 17.1 kg/m3 of water-dispersible fibers having a length between about 10 and about 25 mm, for instance glass or polymer fibers, to a pumped aqueous base-fluid including solid particles having an equivalent diameter of less than about 300 microns. Another known process utilizes melt-processed inorganic fibers selected from basalt fibers, wollastonite fibers, and ceramic fibers.
Such known methods and compositions, however, do not effectively control fluid loss if low or moderate amounts of fibers are used. Therefore, a method for controlling loss and utilizing moderate and low levels of fibers is desirable. Moreover, such a process utilizing sufficiently small sized fibers so as to be consistent with downhole assemblies would be desirable.